In the chemical synthesis of naturally occurring substances having physiological activities or analogues thereof, a method for selective replacement of the OH group by hydrogen has become increasingly important in recent years, and various methods have been developed so far. (Reference: e.g., Tetrahedron Vol. 39, No. 16, pp. 2609.about.2645 (1983))
Methods for deoxygenation of alcohols of this type that have been reported to date may be roughly classified as follows.
A. A method which involves nucleophilic replacement of the OH group in the alcohol by a halogen or a mercapto group followed by reductive dehalogenation or desulfurization (hereinafter referred to as Method A);
B. A method which involves introduction into the OH group in the alcohol of a tosyl, mesyl, sulfo or O-alkylisourea group followed by reduction (hereinafter referred to as Method B); and
C. A method which involves introduction into the OH group in the alcohol of (a) an O-alkylthiocarbonyl, chloroformyl, phenylselenocarbonyl or benzoyl group to add a radical, (b) a carbonyl, thiocarbamoyl, phosphono, phosphoamidoyl, sulfo or mesyl group to cause an electron-transfer reaction, or (c) a carbonyl, thiocarbonyl, trifluoromethanesulfonyl or thiocarbamoyl group, for example, to cause photoexcitation to form an intermediate radical which, in turn, undergoes .beta.-cleavage (hereinafter referred to as Method C).
Some examples of known methods for deriving deoxyribonucleosides from ribonucleosides according to Method C are as follows.
(1) After the 3'- and 5'-position OH groups of a ribonucleoside have been protected with 1,1,3,3-tetraisopropyldisiloxane, a phenoxythiocarbonyl group is introduced into the 2'-position OH group thereof, and, through the use of 2,2'-azobisisobutyronitrile (AIBN) as an initiator, reduction is carried out with tri-n-butyltin hydride, deprotection being then carried out to obtain a 2'-deoxyribonucleoside. (Reference: J. Am. Chem. Soc., Vol. 105, pp. 4059.about.4065 (1983))
(2) After the 3'- and 5'-position OH groups of a ribonucleoside have been protected with a benzoyl group, a thiobenzoyl group is introduced into the 2'-position OH group thereof, and cleavage is carried out with tri-n-butyltin to obtain a 2'-deoxyribonucleoside. (Reference: J. Org. Chem., Vol. 46, pp. 4301.about.4304 (1981))
Among conventional methods for deoxygenation of alcohols, Methods A and B, for example, which are based in principle on ionic reactions so that reactants or intermediates are liable to be easily solvated, cannot be applied to complicated, polyfunctional compounds having sterically hindered OH groups. Further, these methods generally entail a side reaction in the substitution reaction or an elimination reaction and thus have problems of a reduction in yield of the desired compound and complicated purification procedures.
In Method C, which is based on a radical reaction, radicals are less solvated in comparison to the above stated two methods, and thus this method has an advantage in that substantially no steric hindrance occurs. Method C, however, gives rise to a pollution problem because reagents used in the reaction are toxic. Other problems with Method C are the need for using expensive reagents and that the reaction must be carried out under strictly water-free conditions.
An object of the present invention is to provide a novel method for deoxygenation of alcohols in which the problems accompanying the conventional methods have been solved, and which is therefore more suitable for industrial applications.